The UConn Plant Diagnostic Lab (https://plant.lab.uconn.edu/) receives many samples of rhododendrons as well as emails from clients wondering why their plants have dead stems and dieback. Rhododendrons are very common landscape plants loved by many homeowners, but they can have a few disease issues. One common disease that we see frequently is Botryosphaeria dieback.

Botryosphaeria dieback is caused by the fungal pathogen, Botryosphaeria dothidea. Symptoms include wilting leaves that roll inwards along the midvein with dead leaves remaining attached to stems. Leaves begin to turn dark green to brown and eventually die because stem cankers block and girdle the stem. Reddish-brown and sunken cankers are found on stems and branch tips. Stem discoloration can extend to the wood and pith, and a cross section of the stem often shows a darkened wedge pointed towards the center of the stem. The infection can extend down branches into the main stem, killing the plant. On older plants, cankers and dead wood can develop black pycnidia, which are fruiting structures of the fungus.

Botrysphaeria dieback on rhododendron. Photo by Elizabeth Bush, Virginia Polytechnic Institute & State University, Bugwood.org

Infections can occur through natural wounds, pruning wounds, leaf scars, or dead branches. Exposure to conditions such as heat or drought stress can make plants more susceptible to infection. Plants exposed to full sun are frequently infected.

Botryosphaeria can also be confused with symptoms of phytophthora infection. Botryosphaeria cankers usually develop more slowly than phytophthora cankers. Phytophthora cankers can extend to the leaves and form brown wedges of discoloration at the bases of leaves and down the midvein. Winter injury symptoms can also look like Botryosphaeria, including tip dieback and leaves rolled along the midvein. Young transplants that have not been acclimated to winter conditions or entered dormancy are often affected by winter injury. Winter injury can be avoided by planting cold tolerant cultivars and following good cultural practices.

Note the reddish color of stem tips and leaves along the midvein. Photo by David L. Clement, University of Maryland, Bugwood.org

Planting site characteristics such as light exposure and soil drainage are important things that diagnosticians consider when a plant is showing symptoms of disease. It is important to know how often your rhododendron plants are getting watered and if the soil has good or poor drainage. Proper irrigation and fertilization can help prevent plants from becoming susceptible to infection. Rhododendrons should be planted in partial sun and watered during periods of drought. Fertilizer applications in late summer or early fall can cause plants to push out new growth that does not harden off properly for winter conditions. These stems will likely suffer from cold injury and become entry sites for infection in the spring.

Managing Botryosphaeria dieback includes the preventative controls listed above. Additionally, all diseased or dead tissue should be pruned away during dry weather. Make pruning cuts at least 6 inches below where symptomatic tissue ends and sanitize tools between cuts with 10% bleach solution or 70% alcohol. Remove all diseased tissue to prevent the disease from overwintering or spreading to healthy plants. Either place in trash or burn it. Following good cultural practices is always recommended when managing disease. Avoid overhead or sprinkler irrigation, as this causes prolonged leaf wetness and aids in disease spread. Avoid wounding plants and encourage good air circulation by clearing the area around plants of any debris or weeds. Sanitation and cleanliness of the planting site is important and often overlooked. Remove any leaves or branches that have fallen to the ground throughout the growing season and do one last sweep in the fall. Fungicide applications are not effective or recommended for use against this disease, so preventative and cultural controls are most important for management.

Lillian Borbas